tests: Update extra repos in different distros.

* epel removed from RHEL-7
* CRB split in RHEL 8/9 vs CentOS due to different naming

tests/build-gating.fmf

@@ -35,19 +35,17 @@ adjust:
         how: shell
         script: dnf config-manager --set-enabled rhel-CRB
     when: >-
-      distro == centos
-      or distro == rhel-9
+      distro == rhel-9
       or distro == rhel-8
 
-  # Unfortunatelly, TMT does not support more declarative approach, we need to run commands on our own.
-  - because: "On RHEL-7, EPEL must be enabled to provided rarer packages"
+  # Unfortunately, TMT does not support more declarative approach, we need to run commands on our own.
+  - because: "On CentOS, CRB must be enabled to provide rarer packages"
     prepare+:
-      - name: Enable EPEL
+      - name: Enable CRB
         how: shell
-        script: |
-          rpm -q epel-release || yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
-          yum-config-manager --enable epel
-    when: distro == rhel-7
+        script: dnf config-manager --set-enabled crb
+    when: >-
+      distro == centos
 
 discover:
     - name: clang-tests